Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for tracking an electromagnetic (EM) receiver's position/orientation and/or calibrating the same.
Discussion of the Background
EM surveying is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry. EM surveys may be based on a controlled source that sends EM energy waves into the earth. By measuring the associated secondary fields with an EM receiver, it is possible to estimate the depth and/or composition of the subsurface features. These features may be associated with subterranean hydrocarbon deposits.
An airborne EM survey system 100 generally includes, as illustrated in FIG. 1, a transmitter 102 for generating a primary electromagnetic field 104 that is directed toward the earth. When the primary EM field 104 enters the ground 108, it induces eddy currents 106 inside the earth. These eddy currents 106 generate a secondary electromagnetic field or ground response 110. An EM receiver 112 then measures the response 110 of the ground. Transmitter 102 and receiver 112 may be connected to an aircraft 114 so that a large area of the ground is swept. Receiver 112 may be located concentric with transmitter 102. The currents induced in the ground are a function of the earth's conductivity and of course, the transmitter characteristics. By processing and interpreting the received signals, it is possible to study and estimate the distribution of conductivity in the subsurface. The distribution of conductivity is associated with the various layers 116 and 118 making up the subsurface, which is implicitly indicative of the location of oil and gas reservoirs, and/or other resources of interest for the mining industry.
EM systems can be either frequency-domain or time-domain. Both types of systems are based on principles encapsulated in Faraday's Law of electromagnetic induction, which states that a time-varying primary magnetic field will produce an electric field. For airborne systems, the primary field is created by passing a current through a transmitter loop (or series of transmitter loops). The temporal changes to the created or radiated magnetic field induce electrical eddy currents in the ground. These currents have an associated secondary magnetic field that can be sensed, together with the primary field, by a series of receiver coils.
Each receiver coil may consist of a series of wire loops, in which a voltage is induced proportional to the strength of the secondary electromagnetic field from the eddy currents in the ground and their rate of change with time. Typical receiver coils have axes in the three Cartesian directions that are orthogonal to one another. Coils with their axes in the same direction as the transmitter coil axis are most sensitive to horizontal layers and half-spaces if the transmitter coil is horizontal. Coils with their axes orthogonal to the horizontal ground are most sensitive to discrete or vertical conductors.
In frequency-domain systems, the time-varying transmitter signal is a sinusoidal waveform of constant frequency, inducing electrical currents in the ground of the same frequency. Most systems use several constant frequencies that are treated independently. Although the secondary field has the same frequency as the primary field, it will have a different amplitude and phase. It is customary to separate the secondary response into two components: in-phase and quadrature. The in-phase component is defined as having the same phase as the transmitter waveform, whereas the quadrature component is shifted in phase by 90° with respect to the in-phase component.
For time-domain systems, a time-varying field is created by a current that may be pulsed. The change in the transmitted current induces an electrical current in the ground that persists after the primary field is turned off. Typical time domain receiver coils measure the rate of change of decay of this secondary field. The time-domain transmitter current waveform repeats itself periodically and can be transformed to the frequency domain where each harmonic has a specific amplitude and phase.
During survey flying, an airborne EM system generally attempts to maintain a certain transmitter altitude and a receiver altitude above the topography. As the terrain changes, the aircraft needs to adjust to maintain constant altitude with respect to the ground, thereby producing inconsistency in the speed and attitude of the aircraft.
Because the receiver is towed by the aircraft, its position, relative to the aircraft, is also altered. Variations in the transmitter and receiver geometry manifest themselves as position changes in each of the x-, y-, and z directions. Changes in the attitude of the transmitter have a similar effect to moving the position of the receiver. For example, if the aircraft was to pitch its nose down, it would have a similar effect on the primary field as moving the receiver some distance closer to the aircraft along the x-direction, thereby changing the amount of primary field measured at the receiver. Thus, position variations of the EM receiver are a source of EM radiation, which is essentially noise. Only a portion of the total noise measured by the EM receiver is caused by changes in orientation and/or position of the receiver's coils.
Thus, the movement of the receiver can create undesirable noise in the EM measurement data. As a result, some attempts have been made in current EM systems to compensate for the noise/error using receiver position or orientation data.
For example, International Patent Application WO2011/063510 describes an airborne geophysical surveying system with a receiver orientation sensing system. However, this sensing system requires complex integration of many components, including three angular accelerometers, a three-axis fluxgate magnetometer and two axis-tilt sensors. In addition, as the noise is calculated based on independent receiver orientation data, which can be out-of-sync with the EM response, this sensing system may not provide accurate EM measurement compensation and is potentially prone to introducing additional errors.
In the context of subsurface drill guidance and mine rescue, Frederick H. Raab describes, in “Quasi-Static Magnetic-Field Technique for Determining Position and Orientation”, IEEE Transactions on Geoscience and Remote Sensing, Vol. GE-19, No. 4, October 1981, pp. 235-243 (Raab herein), a technique to measure position and orientation based on multi-axis excitation and sensing of quasi-static magnetic fields. However, this technique generally is not readily applied to time-domain EM systems because the tracking signals are additive and can mask the very small signals of interest in the earth response.
Other existing prior art systems for determining object position and orientation, such as those described in U.S. Pat. No. 3,983,474 to Kuipers, and U.S. Pat. No. 4,829,250 to Rotier, generally suffer from complex integration problems for use in airborne EM surveying systems and are not readily applicable in airborne time domain EM systems.
Therefore, there remains a need for an improved system for tracking EM receivers used in EM surveying systems.